This invention relates to the production of light metal alloys having improved combinations of properties. The invention further relates to a method for making light metal-rare earth metal alloys from pellets of light metal powder and rare earth metal-containing compound. More particularly, the invention relates to a method for aluminothermically reducing scandium oxide to form aluminum-scandium alloys therefrom.
In the field of alloy development, research is continuously conducted on methods for improving the behavioral characteristics of existing aluminum, magnesium and other light metal alloys. Additional research is directed to the development of new alloy compositions having desired property combinations. For nuclear and aerospace applications, aluminum-based or magnesium-based alloys are often preferred because of their relatively high strength-to-weight ratios and corrosion resistance. Such alloys could be made more attractive to aerospace product manufacturers if rare earth metals were efficiently and economically incorporated into their compositions. That is because even trace amounts of rare earth metals tend to improve corrosion resistance still further while positively affecting relative alloy density. Minor additions of scandium, for example, are known to improve the tensile and yield strengths of an aluminum alloy according to U.S. Pat. No. 3,619,181. Scandium additions of up to about 10% also contribute to the superplastic formability of aluminum alloy products according to U.S. Pat. No. 4,689,090. Still further improvements may be realized by adding rare earth metals to aluminum brazing alloys (as in U.S. Pat. No. 3,395,001) or by metalliding aluminum surfaces with rare earth metals (as in U.S. Pat. No. 3,522,021). According to Russian Patent Nos. 283,589 and 569,638, scandium additions to magnesium-based alloys improve foundry characteristics, corrosion resistance and/or mechanical strengths.
Although rare earth metal additions improve certain light metal alloy properties, they have not been added to aluminum or magnesium on a commercial scale due, in part, to the difficulty and expense of removing rare earths from the ores containing them. Presently known methods for producing "ingot quality" scandium, for example, require steps for first converting scandium oxide to ScF.sub.3 using hydrofluoric acid, reducing the scandium fluoride to a salt with calcium, then vacuum melting the scandium from this salt. Unfortunately, this production method is rather costly and inefficient. About fifty percent (50%) of the scandium within ore treated by this method is not recovered. In addition, the "ingot quality" scandium alloy that is produced typically contains minor amounts of titanium and/or tungsten which are absorbed from the containers used in the aforementioned recovery method.
In U.S. Pat. No. 3,846,121, an alternative method for producing scandium metal was disclosed which consists of firing scandium oxide in air to remove any volatile residues therefrom; chlorinating air-fired oxides with phosgene; then reducing the ScCl.sub.3 to magnesium-scandium for subsequent purification by vacuum distillation or arc-melting. Once scandium has been isolated from its ore by one of these methods, it must still be alloyed with one or more other metals. Such rare earth metal additions pose their own set of complications, however. If a scandium ingot was added directly to a molten aluminum bath, scandium aluminide intermetallics would first form, said intermetallics having melting temperatures hundreds of degrees higher than those associated with aluminum alone. With the increased presence of these intermetallics, alloy mixing would have to be slowed, thereby resulting in an increased chance of producing inhomogenous alloy products therefrom.
Several direct means for making light metal-rare earth metal alloys are also known. U.S. Pat. No. 3,855,087, for example, codeposits rare earth metal and aluminum (or magnesium) onto a solid molybdenum, tungsten or tantalum cathode rod by simultaneously reducing oxides of both metals in a molten bath containing LiF and preferred rare earth metal fluorides. The alloy that is produced collects in a non-reactive refractory receptacle placed beneath the cathode rod. West German Patent Application No. 2,350,406 shows a similar method for producing light metal-rare earth metal master alloy by electrolytically reducing combinations of light metal oxide and rare earth metal oxide in another fluoride salt bath.
In U.S. Pat. No. 3,729,397, there is claimed a method for making magnesium-rare earth metal alloys by electrolytically reducing rare earth metal oxides in a salt bath using a molten magnesium cathode. Once reduced rare earth metal deposits on the molten cathode confined to a boron nitride sleeve, magnesium-rare earth metal alloy is physically recovered from the sleeve through such mechanical means as ladling, tapping or the like.
French Patent No. 2,555,611 shows a method for reacting rare earth metal oxide with an aluminum powder, preferably under an inert gas cover maintained at atmospheric pressure. When a homogeneous mixture of the aforementioned components is heated at temperatures exceeding 700.degree. C., or well above the melting point for aluminum, an aluminum oxide by-product forms which may be skimmed from the molten alloy surface. In Russian Patent No. 873,692, there is disclosed a method for preparing aluminum-scandium master alloy by combining aluminum powder with scandium fluoride under vacuum in three temperature-increasing stages. Said method is intended to lower the fluoride content of the resulting master alloy.
There are also known several means for premixing certain alloying components or subcomponents. U.S. Pat. No. 2,911,297, for example, claims a process for introducing high melting temperature constituents into molten metal by combining powdered forms of one metal and a dispersing salt in a briquette, said dispersing salt being capable of evolving gases at a sufficient pressure for spontaneously disrupting the briquette following its introduction to the melt. According to the reference, this process may be used for adding pulverized manganese, copper, nickel or chromium to molten metals.
In U.S. Pat. No. 3,380,820, there is shown a method for making aluminum alloys containing between 2-25% iron. The method includes mixing aluminum with iron particles having a maximum dimension of less than one inch, compressing this mixture into a briquette, and melting the briquette before casting it into a desired shape.
U.S. Pat. No. 3,592,637 claims an improved process for making direct metal additions to molten aluminum. The process commences by blending finely-divided aluminum powder with one or more other finely-divided metals selected from: Mn, Cr, W, Mo, Ti, V, Fe, Co, Cu, Ni, Cd, Ta, Zr, Hf, Ag and alloys thereof. Mixtures of these two (or more) metals are then compacted to about 65-95% of their maximum theoretical density. In U.S. Pat. No. 4,648,901, the aluminum and other metal component(s) from the preceding patent were further admixed with a flux of potassium cryolite, potassium chloride, potassium fluoride, sodium chloride, sodium fluoride and/or sodium carbonate before compaction into "tablets".
In U.S. Pat. No. 3,935,004, recovery efficiencies are enhanced by pelletizing aluminum alloying components such as manganese, chromium and iron with up to 2.5% of a non-hygroscopic fluxing salt and, if necessary, a small amount of binder material. Before these alloying components are combined with flux (and binder), they are first reduced to an average particle size less than about 0.25 mm using conventional grinding techniques.
U.S. Pat. No. 3,941,588 shows still other means for incorporating materials into a molten metal bath. Specifically, alloying metals such as manganese or chromium, in particulate form, are admixed with flux and a finely divided phenolic resin, preferably in the form of low density microballoons. The foregoing composition is then added to molten aluminum as a powder or in lump, bag or briquette form. In U.S. Pat. No. 4,171,215, finely divided beta manganese particles are blended with aluminum powder before compaction into readily usable briquettes.